1. Field of the Invention
This invention relates to a pressurized gas dispensing system and more particularly to a method and apparatus useful for transferring compressed natural gas ("CNG") from a refueling station into one or more vehicle storage tanks.
2. Description of Related Art
Systems for compressing and dispensing natural gas are well known, having previously been disclosed for example in U.S. Pat. Nos. 3,837,377; 4,515,516; 4,527,600; 4,966,206; 5,029,622; 5,169,295; 5,238,030; 5,259,424; and 5,351,726.
Because of the interrelationship between the temperature, pressure and volume of gases, the amount of CNG that can safely be introduced into a storage tank such as a vehicle storage tank during refueling necessarily depends upon factors such as the volume and design pressure of the tank, and the temperature and pressure of the gas inside the tank. Industry convention sets the pressure rating for CNG fuel tanks at the standard temperature of 70.degree. F., so nominal pressure ratings such as 2400, 3000 and 3600 psi correspond to an internal gas temperature of 70.degree. F. During rapid refueling, the internal tank temperature will typically rise about 70.degree. F. due to adiabatic compression of the gas. After the tank is filled, the temperature and pressure inside the tank will normally decrease as the gas cools. Wide variations in ambient temperature above or below the standard condition of 70.degree. F. can also have a significant effect on the indicated pressure inside the tank during and after refueling. In addition to safety considerations, the effects of temperature and pressure on the volume of gas dispensed during refueling are also important for billing or cost accounting purposes.
Prior art CNG refueling systems have used various devices and methods for regulating the cutoff pressure and for determining the amount of gas dispensed when refilling vehicle storage tanks. U.S. Pat. No. 3,837,377 discloses means for sensing the pressure of a given amount of reference gas contained in a closed reference pressure vessel that is in thermal contact with the tank being filled. Gas is charged into the tank and the pressure in both the reference vessel and the tank are monitored and compared. Refueling is terminated whenever there is a predetermined pressure differential between the gases in the reference vessel and the tank. The use of an internal reference vessel increases fabrication and installation costs and also presents an ongoing maintenance problem with regard to checking the integrity of the reference vessel. If the reference vessel leaks, there is no way to verify that the reference pressure is correct and has not changed. Also, the use of a reference vessel as disclosed in U.S. Pat. No. 3,837,377 will not provide a desirably quick response time in that the gas flow rate will be gradually lowered as the receiving tank pressure begins approaching the reference pressure.
U.S. Pat. No. 4,527,600 discloses a CNG dispensing system comprising a relatively high pressure storage tank from which CNG flows through a control valve, pressure regulator and flow sensing transducer to the tank being filled. Temperature and pressure transducers in the storage tank transmit electric signals to a process control computer that calculates the volume of gas dispensed by comparing the initial and final values of the CNG inside the storage tank. A differential pressure cell communicating with the storage tank and with the vehicle tank fill line generates a signal that is used by the computer to operate a solenoid-controlled valve disposed in the fill line. Flow continues until pressure in the vehicle storage tank reaches a preselected set point, causing the regulator to close. The regulator set point is not, however, adjusted according to the temperature inside the vehicle tank.
U.S. Pat. No. 5,029,622 discloses a gas refueling device and method of operation wherein at least one temperature sensor is utilized for sensing the temperature of ambient air external to the refueling device and generating a first actual value signal while the pressure sensor senses the pressure of the gas flow in the gas distribution means and generates another actual value signal in response thereto. An advantage of this refueling device is said to be that set/actual value comparisons between pressures and temperatures are carried out at short time intervals, with the permissible set values being corrected according to the development of the preceding measurements. Here again, however, the temperature inside the vehicle storage tank is not monitored during refueling.
U.S. Pat. No. 4,966,206 discloses another device for refilling tanks with CNG that automatically adjusts the filling pressure of the gaseous fuel to the local ambient temperature. A temperature sensor is disposed outside the casing of the device for generating a signal in response to ambient temperature. A pressure sensor is connected to the suction line of the compressor for generating a signal in response to the gas inlet pressure. A pressure difference sensor is also provided for generating a signal in response to a difference in pressure between the fuel pressure in the inlet line of the casing and the pressure inside the casing. A control device disposed in the casing is connected to each of the three sensors in order to receive signals from each. This control device is also connected to the inlet valve and discharge valve in order to control the inlet valve and discharge valve in response to the signals received from the sensors.
U.S. Pat. No. 5,238,030 discloses a pressurized fluid dispensing system that can automatically compensate for non-standard ambient gas temperature to promote complete filling of a pressurized storage tank. Pressure and temperature transducers connected to the supply plenum measure the stagnation pressure and temperature of the CNG, and a pressure transducer in fluid communication with the vehicle tank via the dispensing hose assembly is used to determine the pressure in the vehicle tank. A second temperature transducer is used to measure the ambient temperature. An electronic control system connected to the pressure and temperature transducers and to the control valve assembly calculates a vehicle tank cut-off pressure based on the ambient temperature and on the pressure rating of the vehicle tank that has been pre-programmed into the electronic control system and automatically turns off the CNG flow when the pressure in the vehicle tank reaches the calculated cut-off pressure.
U.S. Pat. No. 5,259,424, related to U.S. Pat. No. 5,238,030, discloses a similar system in which the pressure transducer is used to determine the discharge pressure; in which the electronic control system calculates the volume of the vehicle tank and the additional mass of CNG required to increase the tank pressure to the cut-off pressure; and in which the CNG flow is turned off when the additional mass has been dispensed into the vehicle tank.